Paul Burridge wrote:

Hi all,

I wanted to build an RF relative field strength meter, so set about
searching on the Web for any existing designs. Those I turned up
weren't particularly impressive, so I decided to start from scratch
and design my own. I've just completed that this afternoon. I've
allowed for 0.25mV input to give rise to FSD on the microammeter.
Question being, however, is that going to be sensitive enough?

Yes. If it's not, either move closer, or add an op amp with
a gain control after the diode(s) & cap. Mine uses 2 gain
controls - 1 on the input to the op amp, and one that sets
the gain of the op amp. A further improvement is the use
of a cheap DPM - no parallax and a wider "full scale" range
with no loss of sensitivity. The downside of the cheap DPM
was the need for 2 9 volt batteries. (I have since built a
small DC-DC converter that occupies the volume of a single
9V battery. That may allow using a single rechargeable
9V to power the converter - but I haven't tried it yet
to be sure that the converter doesn't produce noise that
would be detected by the instrument. The converter does
produce dual 9V fully isolated outputs at at least 10 ma
per output, way more than the instrument needs.)


Does
anyone have any idea what the field strength in microvolts or
millivolts is from a half Watt transmitter at about 6 feet away? I
guess I should have posed this question *before* designing it, but who
among us can honestly say they haven't designed something without
knowing what the spec is? :-)
Anyway, ballpark figures gentlemen, please.

p.
--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill